Semiempirical Magnetostructural Correlation for High-Nuclearity MnIII-Oxo Complexes: Accommodation of Different Relative Jahn-Teller Axis Orientations.

The previous development of a magnetostructural correlation (MSC) for polynuclear FeIII/oxo clusters has now been extended to one for polynuclear MnIII/oxo clusters. A semiempirical model estimating each pairwise Mn2 exchange constant (Jij) from the Mn-O bond lengths and Mn-O-Mn angles has been formulated based on the angular overlap model. The extra complication, compared with the FeIII/oxo MSC, of different relative orientations of the Jahn-Teller distortion axes typical of high-spin MnIII in near-octahedral geometry was accommodated by developing a separate MSC variant for each possible situation. The final coefficients of the three MSC variants were determined by using reliable crystal structure data and experimentally determined Jij values from the literature. The estimated JMSC values from the new MnIII/oxo MSC have been employed to successfully rationalize the magnetic properties of a number of MnIII clusters in the nuclearity range Mn3-Mn10. These properties include relative spin vector alignments in the ground state, the presence of spin frustration effects, and the resulting overall ground state spin. In addition, the JMSC values can be used to simulate the direct-current magnetic susceptibility versus temperature data and provide realistic input values for fits of these data to minimize false-fit problems. A protocol for the use of the new MSC is also reported.

Synthesis, Structure, and Magnetic Properties of an Fe36 Dimethylarsinate Cluster: The Largest "Ferric Wheel".

The synthesis and characterization of a high-nuclearity FeIII/O/arsinate cluster is reported within the salt [Fe36O12(OH)6(O2AsMe2)63(O2CH)3(H2O)6](NO3)12 (1). The compound was prepared from the reaction of Fe(NO3)3·9H2O, dimethylarsinic acid (Me2AsO2H), and triethylamine in a 1:2:4 molar ratio in acetonitrile. The Fe36 cation of 1 is an unprecedented structural type consisting of nine Fe4 butterfly units of two types, three {FeIII4(µ3-O)2} units A, and six {FeIII4(µ3-O)(µ3-OH)} units B, linked by multiple bridging Me2AsO2- groups into an Fe36 triangular wheel/loop with C3 crystallographic and D3 virtual symmetry that looks like a guitar plectrum. The unusual structure has been rationalized on the basis of the different curvatures of units A and B, the presence of intra-Fe36 hydrogen bonding, and the tendency of Me2AsO2- groups to favor µ3-bridging modes. The cations stack into supramolecular nanotubes parallel to the crystallographic c axis and contain badly disordered solvent and NO3- anions. The cation of 1 is the highest-nuclearity "ferric wheel" to date and also the highest-nuclearity Fe/O cluster of any structural type with a single contiguous Fe/O core. Variable-temperature direct-current magnetic susceptibility data and alternating-current in-phase magnetic susceptibility data indicate that the cation of 1 possesses an S = 0 ground state and dominant antiferromagnetic interactions. The Fe2 pairwise Ji,j couplings were estimated by the combined use of a magnetostructural correlation for high-nuclearity FeIII/oxo clusters and density functional theory calculations using broken-symmetry methods and the Green's function approach. The three methods gave satisfyingly similar Ji,j values and allowed the identification of spin-frustration effects and the resulting relative spin-vector alignments and thus rationalization of the S = 0 ground state of the cation.

Iron(III)-Oxo Cluster Chemistry with Dimethylarsinate Ligands: Structures, Magnetic Properties, and Computational Studies.

A program has been initiated to develop FeIII/oxo cluster chemistry with the "pseudocarboxylate" ligand dimethylarsinate (Me2AsO2-) for comparison with the well investigated FeIII/oxo/carboxylate cluster area. The synthesis and characterization of three polynuclear FeIII complexes are reported, [Fe12O4(O2CtBu)8(O2AsMe2)17(H2O)3]Cl3 (1), Na2[Fe12Na2O4(O2AsMe2)20(NO3)6(Me2AsO2H)2(H2O)4](NO3)6 (2), and [Fe3(O2AsMe2)6(Me2AsO2H)2(hqn)2](NO3) (3), where hqnH is 8-hydroxyquinoline. The Fe12 core of 1 is a type never previously encountered in FeIII carboxylate chemistry, consisting of two Fe6 units each of which comprises two {Fe3(µ3-O2-)} units bridged by three Me2AsO2- groups and linked into an Fe12 loop structure by two anti-anti η1:η1:µ Me2AsO2- groups, a bridging mode extremely rare with carboxylates. 2 also consists of two Fe6 units, differing in their ligation from those in 1, and this time linked together into a linear structure by a central {Na2(NO3)2} bridging unit. 3 is a linear Fe3 complex with no monatomic bridges between FeIII ions, a very rare situation in FeIII chemistry with any ligands and unprecedented in Fe carboxylate chemistry. The distinct differences observed in arsinate vs carboxylate ligation modes are rationalized largely based on the greater basicity of the former vs the latter. Variable-temperature dc and ac magnetic susceptibility data reveal all Fe2 pairwise interactions to be antiferromagnetic. For 1 and 2, the different Jij couplings were estimated by use of a magnetostructural correlation for high nuclearity FeIII-oxo clusters and by density functional theory calculations using broken symmetry methods, allowing identification of their relative spin vector alignments and thus rationalization of their S = 0 ground states. The Jij values were then used as input values to give excellent fits of the experimental χMT vs T data. For 3, the fits of the experimental χMT vs T data to the Van Vleck equation or with PHI gave a very weak J12 = -0.8(1) cm-1 (H = -2JSi·Sj convention) between adjacent FeIII ions and an S = 5/2 ground state. These initial FeIII arsinate complexes also provide structural parameters that help validate literature assignments of arsinate binding modes to iron oxide/hydroxide minerals as part of environmental concerns of using arsenic-containing herbicides in agriculture.

Thermal Transport in Supported Graphene Nanomesh.

Graphene is considered as a promising candidate material to replace silicon for the next-generation nanoelectronics because of its superb carrier mobility. To evaluate its thermal dissipation capability as electronic materials, the thermal transport in monolayer graphene was extensively explored over the past decade. However, the supported chemical vapor deposition (CVD) grown monolayer graphene with submicron structures were seldom studied, which is important for practical nanoelectronics. Here we investigate the thermal transport properties in a series of CVD graphene nanomeshes patterned by a hard-template-assisted etching method. The experimental and numerical results uncovered the phonon backscattering at hole boundary (<100 nm neck width) and its substantial contribution to the thermal conductivity reduction.